Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

The description discloses the key side effect ('setting a new primary unsets the previous one') which goes beyond the annotations' destructiveHint. It also includes explicit response formatting and data accuracy contract requirements, adding significant behavioral context not present in the annotations.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is well-structured with clear sentences. The core purpose is stated first, followed by behavioral details. However, the appended 'Data accuracy contract' and 'Powered by CorpusIQ' boilerplate add length that is generic and not tool-specific, making it slightly less concise than ideal.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the tool's simplicity (one parameter, no output schema), the description is complete. It covers the action, the effect on other tools, the uniqueness constraint, and the response requirements, leaving no critical gaps for an agent to invoke it correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The single parameter channel_id is fully described in the schema ('Channel ID (UC...) of an already-registered brand to mark as primary'). Since schema coverage is 100%, the description adds no additional meaning beyond what the schema already provides, warranting the baseline score.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool's function with the verb 'Change' and identifies the resource as the user's primary YouTube Brand Account. It also explains the consequence (becoming default target for my_youtube_* tools) and distinguishes it from adding/removing brand accounts.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description explains when the primary setting matters ('default target for per-channel my_youtube_* tools when no channel_id is supplied'), giving contextual guidance. However, it does not explicitly contrast with sibling tools like add_my_youtube_brand_channel or remove_my_youtube_brand_channel, so the 'when to use vs alternatives' is implied rather than stated.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.